Superheater for chemical recovery units



Aug. 22, 1950 A. L. HAMM 2,519,566

SUPERHEATER ECR CHEMICAL RECOVERY UNITS Filed Nov. 10, 1945 2 Sheets-Sheet l Aug. 22, 1950 A. L. HAMM SUPERHEATER FOR CHEMICAL RECOVERY UNITS 2" sheets-sheet 2 Filed Nov. 10, 1945 Patented ug. 2K2, 1950 SUPERHEATER FOR CHEMICAL RECOVERY UNITS Alexander L. Hamm, New York, N. Y., assgnor to Combustion Engineering-Superheater, Inc., a corporation of Delaware v Application November 10, 1945, Serial No. 627,853

s claims. (c1. 12a-476)" This invention relates to superheaters and is particularly applicable to chemical recovery furnaces such as those in which chemical is recovered from the black liquor of wood pulp.

Black liquor having been heated to a predetermined temperature and raised to a predetermined density by evaporation is sprayed under pressure into the space in the furnace where the heat from the burning combustible constituents in the liquor is high enough to dry and partly volatilize the liquor. Part of the combustion occurs in this space but the greater part takes place on the hearth at the bottom of the furnace onto which the dried portions of the sprayed liquor fall as socalled ash. The combustion in the furnace occurs preferably in the presence of heat absorbing surfaces, such as water cooled tubes forming a part of a steam generator. The recovery units are self sustaining, that is, there is a commensurate amount of heat carried into the furnace by the black liquor to compensate both for the heat given up to the absorbing surfaces and that required for the successful drying and burning of the combustible constituents on the hearth.

The products of combustion rising from the fuel bed on the hearth entrain chemicals, mostly sublimated sodium salts, which yielding heat to the heat absorbing surfaces of the furnace condense into plastic and eventual solid states. The globules or particles of condensed chemicals occasion difficulties by imprinting upon and building up on the tubes of the heat absorbing surfaces which are located within the space of the furnace through which the gases flow on their way to the boiler convection bank.

In my U. S. Patent No. 2,319,399 I show a, preferred form of recovery furnace in which the arrangement of the tubes is particularly advantageous with respect to the gas flow within the furnace and the gas temperatures, or with respect to the state of the globules of the chemical rising with the gases.

An object of this invention is to provide an improved arrangement of superheater tubes within such a recovery furnace.

Figure 1 is a, sectional elevation of a recovery furnace embodying the invention, taken on line I-I of Figure 2.

Figure 2 is a partial section taken on line 2-2 of Figure 1.

Figure 3 is a plan section taken on line 3-3 of Figure 1.

Figures 4 and 5 are elevations in diagrammatic form showing different arrangements of the superheater of Figure 1.

Referring now to Figure 1, the reference character A denotes a smelter furnace, having its walls lined with finned tubes I2, connected with the boiler proper for circulation, as will further appear. Immediately above the smelter furnace A is a chamber B to the rear of which is the boiler I4. The furnace chamber A and the chamber B are of substantially equal height and have an overall height about sixty feet. The tubes I2 are approximately thirty feet long in the installation shown. v

Black liquor evaporated to av desired density of approximately to 70% solids is introduced into the furnace chamber B by conventional nozzles I5 at a pressure from 15 to 25 pounds per square inch and ata temperature of from 220 deg. to 240 deg. F., as described for example in U. S. Letters Patent No. 2,213,052, issued August 27, 1940. The sprayed particles gravitate in counterfiow with the rising flame and gas stream from the smelting zone and are dried in space. The dried particles collect on the hearthIG in the form of a bed 0f fuel I'I. Preheated air for reduction is admitted through the fuel bed by means of the air noz'zles I8, in an amount sufcient to maintain a reducing atmosphere in the fuel bed. The chemical flows out in molten form from the hearth. The combustibles in the gases rising from the fuel bed I1 are burned, for which purpose additional preheated air is admitted above the fuel bed by means of the air nozzles 20, preferably downwardly inclined. Combustion under normal conditions of operation is usually completed at about the bottom of upper chamber B or somewhat thereabove.

The nned tubes I2 in effect constitute a continuous metallic wall lining the interior of the furnace and the inner face of which is exposed to radiant heat. The heat absorbedby the exposed surfaces is sufficient to lower the temperature of the gases leaving the top of the chamber A and entering the bottom of the chamber B to approximately 1800 to 2200 deg. F., well above the melting point of the chemicals. These two chambers open into one another at the level of the headers 22 and 24 into which the upper ends of the tubes I2 constituting the side and front walls of the inner walls of the furnace are expanded.

The boiler proper, comprises an upper steam and water drum 30, connected by banks of upright tubes I4 and to a lower drum 32, the tubes of the rows being in staggered arrangement. 4The banks are baffled to provide vertical passes, the entrance to the first pass being below the steam and Waffel drum 30. The front baffle 34 is preferclosed. These tubes. the fins, and the closure members thus not only constitute a baille for the first bank of tubes but alsol form the rear of the chamber B.

The front and side walls of the chamber B are also lined with finned tubes 83 so that the chamber B is lined with a substantially continuous metallic wall, the inner face of which is exposed to radiant heat.

Within chamber B is a vertically disposed superheater 40 suspended from above, comprising parallel rows of tubes connected in parallel to inlet header 42 and outlet header 43. The alternate rows of superheater tubes are aligned transversely with respect to the boiler drum 30, and

depend into chamber B for di'erent distances,

tubes 44 depending to about half the way down and tubes 46 to about three quarters. By this arrangement the spacing between said rows in direction parallel to the drum 30, is about twice as great in the lower portion of chamber B than in th upper portion. The distances to which the long and short rows depend is determined by the state of the globules or particles of sodium salts condensing out of the products of combustion rising upwardly through the furnace. These sodium salts have been sublimated from the so called ash bed on the hearth of the furnace due to the prevailing flame temperatures of about 1800 to 2200 deg. F. The predominant salts, sodium carbonate and sodium sulphate, melt at about 1400 deg. F. above which they are successively in a semiplastic and vapor state and below which, to about 1200 'deg. F., they may be sulciently plastic to adhere to tubes when flowing thereacross. To obtain these salts in a fine dry crystalline state, it is necessary to reduce the temperature of the products of combustion and the contained salts to temperatures below about 1400 deg. F. Obviously this temperature may vary with different salts. At temperatures below about 1400 deg. F. the superheater tubes may be more closely spaced than above said temperature. Also, as disclosed in my prior patent above mentioned, it is necessary for satisfactory operation that the tubes be arranged in the upper chamber B so the gases will ow parallel thereto for a distance until the gases have been cooled to not over 1200 deg. F., and thereafter the gases may flow transversely to the tubes.

The superheater 40 embodies all of the above advantages. Where it is exposed'to gas temperature of about 1350 deg. and above the tubes 46 are widely spaced and below this temperature the tubes 44 are closely spaced. The widely spaced tubes 46 permit a substantial portion of the gases to flow longitudinally therebetween without contact thereby minimizing condensation of the salts thereon.

Because alternate rows of tubes are of different lengths, half of the tubes 44 of one row are serially connected to half of the tubes 46 of a next adjacent row by means of crossover tubes 48, thereby making the length of steam travel, and therefore the resistance, from inlet header 42 to outlet header 43 through all of the parallel circuits substantially the same. Crossover tubes 48 are located above the bafile 36 extending between the steam and water drums 30 and 38 and are thereby protected from the heat of the products of combustion.

In Figure 4 the superheater has rows of alternately short and long tubes, each IOW having all '4 of its tubes serially connected for continuous flow of fluid therethrough. Each row of short tubes is connected to an inlet header 50 at one end and each long row connected to an outlet header 52 at one end. The opposite ends are then connected to a third common header 54. The short tubes may be made of smaller diameter to compensate by increased resistance for the greater resistance of the longer tubes, or may be made of the same size tubes andprovided with resistances such as orifices either in the tubes or in a header. Alternatively, the tube rows may be made up as shown in Figure 5, with half of the tubes in a row being short tubes 60 and half long tubes 62. By arranging alternate row with the long tubes 62 to the right and intermediate rows lwith the long tubes 62 to the left, the desired wider spacing of the tubes 62 in the lower zone is accomplished. There may be a common header 63 to which the inner ends of both the short and long tube groups are connected.

While I have shown and described some forms of my invention, it will be understood that minor changes in construction, combination and arrangement of parts may be made without departing from the spirit and scope of the invention as claimed.

What I claim is:

1. In a system for recovering chemical and generating steam from the black liquor of wood pulp mills having a smelter furnace into which black liquor is sprayed with a chamber thereabove formed with an opening at the upper part through which the gases flow laterally into the tube bank of the boiler, and closely spaced exposed heat absorbing tubes lining said furnace and the walls of said chamber; other heat absorbing tubes within the chamber and in advance of the boiler and connected to the boiler for circulation of boiler steam therethrough and the amount of exposed surface of the said tubes being sufllcient to lower the temperature of the gases before entering the boiler at all regular self-sustaining rates of operation to not over 1200 deg. F., said other heat absorbing tubes forming a unitary superheater assemblage so disposed within the chamber that the gases have substantially parallel flow with the tubes, certain rows of said superheater tubes depending into the chamber for substantially longer distances than do other rows of shorter tube elements whose positions in the assemblage alternate with the positions of the aforesaid rows of longer tube elements, the said longer tube-element rows extending downwardly into a zone of gas temperatures above about 1400 deg. F. and the said shorter tube-element rows extending for lesser distances only into a zone of gas temperatures below about 1400 deg. F. whereby the spacing between superheater tube rows is substantially greater in the zone at gas temperatures above about 1400 deg. F. than it is in the zone at gas temperatures therebelow.

2. In a system for recovering chemical and generating steam from the black liquor of wood pulp mills which system comprises a smelter furnace into which the black liquor is sprayed, a chamber thereabove successively arranged with respect thereto as to flow of combustion products and having a discharge opening in the upper chamber portion remote from the furnace, and a boiler successively arranged with respect to the chamber and having a tube bank which receives the combustion products as same pass out of said chamber opening, the combination of heat-absorbing superheater tubes within said chamberl connected with said boiler and there so positioned between the boiler and furnace that the combustion products passing through the chamber have substantially parallel flow with respect to the axes of those tubes, said superheater tubes being organized into a unitary superheater assemblage that is made up of sections of relatively long serially connected tubes and of interposed sections of relatively short serially connected tubes all disposed to form relatively uniformly spaced rows with the long tube sections extending a substantial distance further toward the furnace than do the short tube sections and thus being more widely spaced apart at their furnace-approaching ends than are adjacent rows of long and short tubes at their opposite-end portions remote from the furnace, the aforesaid superheater tube positioning being such that said combustion products in flowing from the furnace to the boiler iirst contact the superheaters widely spaced furnaceend tube portions at comparatively high temperature and by those portions are progressively cooled to a reduced temperature before reaching the more closely spaced opposite-end tube portions nearest said chambers discharge opening.

3. In a system for recovering chemical and generating steam from the black liquor of wood pulp mills which system comprises a smelter furnace into which the black liquor is sprayed, a chamber thereabove successively arranged with respect thereto as to flow of combustion products and having a discharge opening in the upper chamber portion remote from the furnace, and a boiler successively arranged with respect to the chamber and having a tube bank which receives the combustion products as same pass out of said chamber opening, the combination of heat-absorbing superheater tubes within said chamber connected with said boiler and there extending vertically downward toward said furnace from the chambers upper portion whereby the combustion products passing upwardly through the chamber will have substantially parallel flow with respect to those tubes, said superheater tubes being organized into a unitary superheater assemblage comprising spaced rows of alternately long and short tube portions serially connected for boiler steam flow therethrough with the long tube portions extending a substantial distance further down in the chamber than do the short tube por-v tions and thus being more widely spaced apart at their lower ends than are adjacent rows of long and short tube portions at their upper ends, the aforesaid superheater tube positioning being such that said upflowing combustion products first contact the superheaters widely spaced lower tube portions at comparatively high temperature and by those portions are progressively cooled to a reduced temperature before reaching the superheaters more closely spaced upper tube portions nearest said chambers discharge opening.

4. In a system for recovering chemical and generating steam from the black liquor of wood pulp mills in which system combustion products from a smelter furnace flow through a chamber and then into the heating surfaces of a boiler, the combination of a superheater positioned within said chamber and comprising tubes organized into a unitary superheater assemblage and arranged in rows parallel to each other and to the flow of combustion products through the chambeiI with the tubes of alternate rows being substantially longer than those in the intermediate rows and withthe tubes in veach alternate and 6 intermediate row constituting continuous uid flow paths from opposite row ends to about the midpoint of each row, an inlet header communieating with said boiler and connected to the inlet ends of the tubes in all of said rows to pass boiler .steam thereinto, an outlet header connected at the outlet ends of the tubes in all of said rows, and a cross-over connection at said midpoint between the long tubes of each alternate row and the short tubes of an adjacent intermediate row whereby each resulting steam flow path from inlet to outlet header includes both long and short tubes connected in series.

5. In a system for recovering chemical and generating steam from the black liquor of wood pulp mills in which system combustion products from a smelter furnace flow through a chamber and then into the heating surfaces of a boiler, the combination of a superheater positioned within said chamber and comprising tubes organized into a unitary superheater assemblage and arranged in rows parallel to each other and to the ow of combustion products through the chamber with the tubes of alternate rows being about twice as long as those in the adjacent rows and about half of the tubes from either end of the row being connected for serial fluid ilow therethrough, the half rows of serially connected long tubes being connected intermediate the ends of the rows to the half rows of the serially connected short tubes in the adjacent row for serial fluid flow through both said halves, an inlet header communicating with said boiler and connected to all of said long and short tube half rows which terminate at the same row end, and an outlet header connected to all of said short and long tube half rows which terminate at the other row end.

6. In a system for recovering chemicals and generating steam from the black liquor of Wood pulp mills which system comprises a smelter furnace into which the black liquor is sprayed, a chamber therebeyond successively arranged with respect thereto as to flow of furnace combustion products and having a discharge opening in the chamber portion remote from the furnace, and a boiler successively arranged with respect to the chamber and receiving the combustion products as same pass out of said chamber opening, the combination of a superheater positioned within said chamber intermediate the furnace and said discharge opening and comprising a bank of tubes organized into a unitary superheater assemblage and arranged in rows parallel to each other and to the ilow of combustion products through the chamber with the tubes of alternate rows being substantially longer than those tubes in the adjacent intermediate rows and with all tubes in each row arranged for serial iiuid flow through the entire tube length, inlet and outlet 'headers one of which is connected to the adjacent ends of the tubes of each of said alternate rows and the other of which is` connected to the adjacent ends of the tubes of each of said intermediate rows, and a third header receiving the other ends of the tubes of each of said alternative rows and of each of said intermediate rows and serving to connect those two sets of tubes in series.

7. In a system for recovering chemicals and generating steam from the black liquor of wood pulp mills in which system combustion products from a smelter furace flow from the furnace into a chamber and then into the heating surfaces of a boiler, the combination of a superheater in said system chamber comprising a bank of tubes organized into a unitary superheater assemblage and arranged in parallel rows for exposure to substantially longitudinal flow of the combustion products from the furnace, the tubes of alternate rows being substantially longer in direction opposite to the iiow of products than the tubes in the adjacent intermediate rows and each row having all of its tubes connected for serial steam flow therethrough, a iirst header connected to one end of each of the alternate tube rows. a second header connected to one end of each of the adjacent intermediate tube rows, a third header connected to the other end of all of said rows of tubes, and a connection from the boiler to at least one of said headers for flowing steam through all of the rows of tubes, said intermediate-row tubes being of smaller diameter than the alternate-row tubes whereby to compensate for the higher resistance to steam Iicw through the longer tubes of the alternate rows.

8. In a. system for recovering chemical and -generating steam from the black liquor of wood said furnace than the tubes in the adjacent rows and all of the tubes in both the longand shorttube rows being connected for serial fluid flow therethrough from opposite row ends to about midway of each row, an inlet header communicating with said boiler and connected to all tubes in each row at the inlet end thereof to pass boiler steam thereinto, an outlet header connected to all tubes in each row at the other end thereof, a cross-over connection between the midpoint of each alternate-row tube coming from one of said headers to the midpoint of an intermediate-row tube coming from the other header, and means protecting all of said cross-over connections from contact by said combustion products.

` ALEXANDER. L. HAMM.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,139,348 Badenhausen Dec. 6, 1938 2,213,052 Rosencrants et al. Aug. 2'7, 1940 2,308,762 Krug et a1. Jan. 19, 1943 2,319,399 Hamm May 18, 1943 FOREIGN PATENTS Number Country Date 589,942 France K Mar. 6, 1925 

